This invention relates to a rotary gear pump of the kind comprising two excentrically arranged gear wheels of different sizes, of which the greater gear wheel is rotatably mounted in the generally cylindrical housing of the pump and the smaller gear wheel is mounted upon a shaft which is arranged excentrically in the housing of the pump in such a way that the teeth of the two gear wheels, in the working condition of the pump, are mutually engaged along an area of the inner circumference of the housing of the pump between the outlet of the pump and the inlet of the pump, and are arranged with an interspace between the top circles of the teeth along another part of the circumferences of the wheels, a blocking member being arranged in the interspace with one surface of the blocking member sealing against teeth of the smaller gear wheel and with another surface of the blocking member sealing against teeth of the greater gear wheel.
A rotary gear pump of this kind is known from French patent specification No. 1528508. According to this prior art one of the gear wheels is arranged so as to be movable in the axial direction of the pump with respect to the other gear wheel in order to create an interspace in the housing of the pump between one side surface of the smaller gear wheel and the opposite side surface of a disc which along its circumference is provided with axially extending projections which form the greater gear wheel. When the pump has been adjusted in this way it allows a flow of fluid to pass through the pump which is far greater than the normal capacity of the pump, viz. the capacity which is achieved due to rotation of the gear wheels when engaged with each other. However, this sort of shiftability requires a long housing for the pump, as seen in the axial direction, because sufficient space must be provided for the axial shifting of one of the gear wheels. Moreover, in order to avoid jamming of the shiftable gear wheel during the shifting it is necessary to provide long guiding means for the movable gear wheel.